How can i download/install the Motion Trail-addon (svn-download)?
While blender has moved it's revision control to git nearly two years ago, you can still find links to the old svn repository, even on blender's own wiki.

While blender's source code is split into four repositories and there are a few other repositories for related projects the one that most users would be interested in is the Add-ons Contrib which can now be found here, where you can also find the git command to download the entire repo. The contrib repo is a collection of semi-official addons that haven't been accepted into the official releases of blender. The old links that you find starting with projects.blender.org or svn.blender.org that have /contrib/ in the path can now be found at developer.blender.org/diffusion/BAC

Now specifically for the addon you mention, the multiple links on the page are different versions of the same addon submitted during it's review process, the addon is contained in one file, the issue you have may be from different versions conflicting or an older version loading first and preventing the new one loading. Remove the older files so that only one new version is installed.

Near the top of an addon script you will find bl_info = { followed by a list of values, the one you are interested in is "version" -

bl_info = {
    ...
    "version": (3, 1, 2),
    ...


only keep the one with the highest version.